(1) Field of the Invention
This invention relates to a spectroscopic apparatus, and more particularly to a spectroscopic apparatus associated with spectroscopic measurement in general, in which spectroscopy is performed.
(2) Description of the Related Art
Spectroscopy means operation of splitting light according to wavelength or frequency, and is utilized for optical measurement in analyzing and measuring the structure of a substance, etc. Spectrometry is widely used in various fields, including medicine and engineering.
Spectrometry includes a pre-splitting method in which light having a continuous spectrum is split and then irradiated onto a sample, and a post-splitting method in which light having a continuous spectrum is irradiated onto a sample without splitting the same and the resulting transmitted/reflected light is split. In both of the methods, the spectroscopic characteristics of a substance can be represented by a spectrum or spectra of light, and therefore, it is possible to study the structure of a substance or chemical phenomena by analyzing the spectrum or spectra of light.
For example, an organic molecule of an organ or the like has a near-infrared absorption characteristic of the molecule, and therefore, near-infrared spectrometry is utilized in analysis of the components of a tissue, by irradiating a near-infrared radiation (having a wavelength of 0.8 to 2.5 μm) onto a sample, and extracting spectroscopic information from the transmitted light or diffuse reflected light.
On the other hand, to split light, an optic called a diffraction grating is generally employed. A spectroscope using the diffraction grating performs splitting of light by irradiating the light onto the diffraction grating to thereby obtain diffracted beams having desired wavelengths.
FIGS. 38 and 39 are diagrams showing diffracted beams formed by the diffraction grating. FIG. 38 shows diffracted beams formed by a diffraction grating G1 having a glass plate formed with e.g. four grooves formed per unit length, while FIG. 39 shows diffracted beams formed by a diffraction grating G2 having a glass plate formed with e.g. eight grooves per unit length.
As can be seen from FIGS. 38 and 39, the magnitude of an opening angle (diffraction angle) between a split diffracted beam having a wavelength λ0 and a split diffracted beam having a wavelength (λ0+Δλ) depends on the number of grooves per unit length. When a diffraction angle θ1 of the diffraction grating G1 having a smaller number of grooves is compared with a diffraction angle θ2 of the diffraction grating G2 having a larger number of grooves, there holds θ1<θ2.
When the diffracted beams are observed, since the diffraction grating G1 has a smaller diffraction angle, for a photodetector 105 to discriminate the respective wavelengths λ0 and (λ0+Δλ), it is required that the distance over which the diffracted beams are transmitted to the photodetector 105 is long. On the other hand, since the diffraction grating G2 has a larger diffraction angle, it is possible for the photodetector 105 to discriminate the respective wavelengths λ0 and (λ0+Δλ), with a shorter distance over which the beams are transmitted.
As described above, by designing the diffraction grating such that the number of grooves per unit length or the order employed is increased, it is possible to increase the diffraction angle. However, in actuality, due to restrictions on machining, there is a limit to increasing the number of grooves. Therefore, conventional spectroscopes have increased the resolution by increasing the distance over which diffracted beams are transmitted.
On the other hand, there has recently been developed an optical branching filter called VIPA (Virtually Imaged Phased Array). The VIPA includes a wavelength dispersion element (VIPA plate) comprised of a glass plate and reflection films coated on both sides of the glass plate. The VIPA receives a collected beam, and causes multiple reflections of the input beam to thereby split the input beam into a plurality of beams having respective wavelengths (see e.g. Japanese Unexamined Patent Publication (Kokai) No. 2000-28849 (Paragraph Nos. [0016] to [0107], FIG. 6).
Further, it has been proposed a technique of making use of the VIPA as a dispersion compensation device for correcting waveform distortions (wavelength dispersion) of an optical signal occurring when it passes through an optical fiber and restoring the signal to its original state (see e.g. Japanese PCT application translation Publication No. 2000-511655, pages 32–33, FIG. 18).
In spectroscopes using the diffraction grating described above, an increase in the resolution results in the increased size of the optical system (optics constituting an optical path over which light generated by a light source travels to a photodetector), which makes it impossible to comply with the demand of reduction of the size of spectroscopes.
Further, the VIPA is capable of wavelength splitting with a larger angular dispersion than the diffraction grating (capable of performing finer light splitting than the diffraction grating), and hence capable of reducing the size of the optical system. Therefore, the VIPA can be applied to a receiver that receives a wavelength-multiplexed signal transmitted by WDM (Wavelength Division Multiplex), whereby it is possible to reduce the size of the receiver and perform compensation for wavelength dispersion. However, it is impossible to apply the proposed VIPA as it is to a spectroscope as an optical measurement apparatus.
It should be noted that the VIPA plate as a component of the VIPA is a period filter and the spectral angle of the beams split by the VIPA plate has a wavelength periodicity. Therefore, in a VIPA-based spectroscope, when an optical signal composed of beams in wavelength bands of λ1 to λn is inputted and each beam is split into light components, one of photoreceptors (PDs: Photo Diodes) constituting a photodetector receives a plurality of light components at the same position within a period of each of the wavelength bands of λ1 to λn.
However, as the optical measurement apparatus, the VIPA-based spectroscope is required to receive a light component (optical spectrum) obtained by splitting one wavelength (band), which makes it impossible to use such a VIPA-based spectroscope as used in WDM as it is, for the spectroscopic apparatus for optical measurement.